U.S. Pat. No. 4,302,250 provides a discussion of tungsten-halogen lamps and their advantage with respect to conventional incandescent lamps. The patent also describes the high temperature parameters under which such lamps operate and the extensive research undertaken to develop glasses demonstrating melting and forming capabilities suitable for mass producing lamp envelopes, as well as exhibiting the physical properties demanded for that application. As illustrative of that prior research, the patent cited U.S. Pat. Nos. 3,496,401, 3,978,362, 4,060,423, and 4,255,198, and noted that, whereas the alkaline earth aluminosilicate glasses disclosed in each of those patents had been fabricated into envelopes for tungsten-halogen lamps, none displayed optimal behavior for that utility either with respect to manufacturing capability or in terms of physical properties.
U.S. Pat. No. 4,302,250 is directed to glasses having melting and forming characteristics suitable for making glass tubing from which envelopes for tungsten-halogen lamps can be fabricated and which can be sealed to molybdenum metal, coupled with the physiochemical properties required in a glass exposed to the operating conditions of a tungsten-halogen lamp. Thus, the glasses of the patent were designed to exhibit a strain point higher than 750.degree. C., a coefficent of thermal expansion (0.degree.-300.degree. C.) between 42-44.times.10.sup.-7 /.degree.C., a liquidus temperature below 1300.degree. C., a viscosity at the liquidus of at least 40,000 poises, and a viscosity of less than 1000 poises at temperatures no higher than 1520.degree. C. Glasses manifesting those properties were prepared from the following straitly-defined ranges of components, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 64-68 BaO 0-5 CaO 11-14 SrO 0-4 Al.sub.2 O.sub.3 16.5-18.5 BaO + SrO 2-6.5 ______________________________________
In the most preferred glasses, both BaO and SrO will be present in a combined amount of at least 3% and in a molar ratio SrO:BaO ranging from about 2:1 to 1:2.
The exceptionally high strain points exhibited by the glasses of that patent are particularly advantageous in enabling lamp operation at temperatures more elevated than possible with the prior art alkaline earth aluminosilicate glasses. Unfortunately, however, the same high temperature capabilities which render those materials extremely attractive for lamp envelope applications also led to manufacturing problems. Specifically, tubing commercially produced therefrom contained an economically unacceptable level of seeds, despite the inclusion of a combined chloride/sulfate fining agent. Thus, alkaline earth aluminosilicate glasses such as are disclosed in that patent (and in the prior art patents discussed therein) have conventionally relied upon the use of chloride and/or sulfate for fining.
As is explained in Pat. No. 4,302,250, the lamp envelopes are fashioned from glass tubing and lead wires of molybdenum metal are sealed into the envelope. Both of these operations require lampworking. Hence, a glass destined to be lampworked must be thermally stable; i.e., it should not be immoderately subject to devitrification, oxidation reboil, and/or deleterious reduction phenomena. Furthermore, the glass ingredients must not chemically react with the halogen atmosphere within the lamp envelope during lamp operation. The use of chloride and/or sulfate for fining the glass has satisfied the above criteria but, as has been noted above, those components have not proved successful in reducing the seed count of tubing prepared from glasses of Pat. No. 4,302,250 to a commercially-acceptable level.
Accordingly, the objective of the present invention was to devise means for improving the fining of such glasses while satisfying the other cited requirements.